Solar energy generation has been a growing field for decades and continues to become more important for power production both on earth and in space. Solar simulators used in characterization and testing of solar cells produce light that represents sunlight (i.e., mimics the emission spectrum of the sun). Measurement systems that use solar simulators are needed in order to understand and validate solar cells in both research and manufacturing. Natural sunlight is rarely used for testing solar cells because it is variable, weather dependent and rarely of the appropriate intensity and spectrum to be used as a standard.
The construction and use of a solar simulator varies depending on the needs of the solar cells being tested. The original solar simulators were used to test simple, single junction solar cells. A junction is a semiconductor region that produces electrical carriers from incident light. Solar cells can be produced that have one junction or that have more than one junction, with the multi-junction solar cells typically having higher performance and higher cost. Although the vast majority of solar cells produced today use a semiconductor junction, solar cells do not need to be made of semiconductor material, as some solar cells are made from dyes, plastics or organic materials. The active sub-cell of a solar cell is called a junction or a sub-cell. A solar cell containing multiple sub-cells is called a multi-junction solar cell and a solar cell containing a single sub-cell is called a single junction solar cell. As used herein, in reference to solar cells, the term “junction”, “sub-cell”, “junction of a solar cell” or “sub-cell of a solar cell” refers to any region of a solar cell that accepts some range of the incident light spectrum and produces electrical carriers from that incident light. Solar cell testing can be for single solar cells or multiple solar cells that are electrically connected together, sometimes called strings or circuits of strings. As used herein, in reference to testing solar cells, the term “solar cell” or “solar cells” can refer to an individual solar cell, a group of solar cells that are not electrically connected together or a group of solar cells that are electrically connected together.
Solar cells for use on earth are typically single junction, and the junction uses doped silicon as the material. Solar simulators for these cells have been developed over many decades, and a variety of products are available. Many of the early terrestrial solar simulators used a xenon lamp as the light source because of the high output power and the spectral similarity between the sun spectrum and xenon output spectrum. Two groups of simulators emerged. With one group, continuous simulators, a relatively small area (typically approximately 1 square foot or less) is illuminated with a continuous beam of light. With the other group, pulsed simulators, a large area (typically approximately 10 square feet or more) is illuminated with a short pulse of light.
Because single junction cells produce current for any light that falls into the acceptance spectrum of the cell, almost any light can be used in solar simulation to illuminate single junction cells. However, solar simulators with spectra that deviate from actual sunlight produce output errors, so the figure of merit for single junction simulators is how well the light output matches a reference solar spectrum. One of the most common reference spectra for terrestrial single junction cells is air mass 1.5, or AM1.5, that represents (i.e., matches) an average sun spectrum and intensity for mid-latitude locations on Earth's surface. This concept of matching a spectrum, AM1.5 or otherwise, is an important element of simulators for testing single-junction, terrestrial solar cells.
The concept of spectral matching was carried forward into other terrestrial solar simulators, including those that do not use lamps and those configured for testing solar cells that have multiple junctions.
Another simulator, which can be used to test terrestrial solar cells uses LEDs instead of lamps as light sources, but still utilizes spectral matching. The terrestrial LED simulators use many different wavelength LEDs, often with overlapping wavelengths, to approximate a piece-wise match of a reference spectrum, such as AM1.5.
Due to the need for increased power, solar cells for space evolved from single junction devices into multi-junction devices. The solar simulators also evolved to support the added junctions. The first space solar simulators were xenon lamps, just like terrestrial simulators. Multiple junctions added additional demands on the simulator, as spectral matching could not be achieved to a high enough performance to give the needed accuracy when testing multi-junction solar cells. The industry created isotype calibration cells to solve this problem. As used herein, “isotype calibration cells” or “isotypes” are multi-junction cells that have all sub-cells electrically inactive except for one. They are intended to be physically identical to a full cell, but with only one working sub-cell. These isotypes can be calibrated by flying them above the atmosphere, exposing them to sunlight, and recording the output current. For some flights, correction factors are also applied. These isotypes can then be used to calibrate solar simulators. Many calibration standards have not actually been flown, but are cloned from calibration cells that have been flown; however, the use is the same.
Given the need to match solar cells with multiple sub-cells, the solar simulators for multi-junction space cells evolved. They still used lamps such as Xenon to match the solar spectrum, but they added adjustment mechanisms to tweak the output spectrum to adjust the amount of current to various isotypes. These adjustments were often highly coupled, but could in some cases be suitably performed by a skilled operator.
Methods to adjust the output of a xenon bulb to give adjustability to the xenon spectrum include adding filters and/or mirrors, often times on a motorized stage. These methods add slight adjustability to the existing lamp (typically xenon lamp) spectrum. Additionally, solar simulators designed for testing solar cells to be used in space are calibrated to the spectrum of light emitted by the sun without passing through any earth atmosphere. This spectrum is commonly referred to as air mass zero or AM0. However, as with solar simulators used to test cells utilized in terrestrial applications, the optical output spectrum of solar simulators designed for testing of cells used in space applications are still configured to match a reference spectrum (which in the case of space applications is AM0).
Like the terrestrial simulators, space simulators have also been formed with LEDs as light emitters. However, the LEDs are still collectively configured to recreate (i.e., match) the AM0 emission spectrum. These LEDs can also perform the function of the mirrors and filters, which is to add slight adjustability to the existing xenon spectrum. There are also systems that attempt to replace the xenon lamp with banks of LEDs, while trying to match the AM0 spectrum.
In addition to using spectral matching, spatial uniformity across the illuminated area is also a figure of merit of a solar simulator. Many methods have been used to achieve good spatial uniformity. Continuous, small area simulators use beam combining methods including filters, mirrors and splitters. Large area, pulsed simulators place the source far from the sample and also use filters and mirrors. One embodiment uses a plate of glass with opaque, filter lines of varying density between the source and the sample solar cell. Simulators with multiple sources can vary the intensity between the sources to improve spatial uniformity.
Once under illumination, may different measurements can be run on the cell. A source measurement unit, or SMU, is well known to one skilled in the art. Many types of tests can be performed, but highly common tests include taking a current versus voltage curve, referred to as an I-V curve, or measuring the short circuit current of the cell (Isc). The I-V curve contains many metrics of interest about the cell under test, including open circuit voltage (Voc), short circuit current (Isc), fill factor (FF), series resistance (Rseries), shunt resistance (Rshunt) and the maximum power point, which is the maximum power voltage (Vmp) and the maximum power current (Imp).
In order to compare simulators, the industry has developed a number of standards for measuring solar simulator performance. These standards include American ASTM E 927-05, European IEC 60904-9 Edition 2 (2007) and Japanese JIS C 8912 standards. These three standards are subtly different, but they each specify requirements for spectral matching, spatial uniformity and temporal stability. Spectral matching is typically specified by the amount of power put into each of approximately 10 spectral bands. If the specified amount of power, plus or minus 10% is put into each of the bands then a simulator is given the highest rating, Class A. For spatial uniformity, the highest rating, Class A, is typically 2% across the illumination plane of interest. For temporal stability the highest rating, Class A, is typically 2%. A simulator that meets the highest standard in all 3 categories, spectral matching, spatial uniformity and temporal stability is called Class AAA.